CHORDATES (脊索动物门）
Intruduction
• Definition（脊索动物的定义）
• The great phylum chordata（脊索动物门） derives its name
from one of the few common characteristics of this group --the
notochord. This structure is possessed by all members of the
phylum, either in the larval or embryonic stages or throughout
life.
• The three distinctive characteristics that chordates apart from
all other phyla are the notochord, dorsal tubular nerve
cord, and pharyngeal gill slits. These characteristics are
always found in the early embryo, although they may be
altered or may disappear altogether in later stages of the life
cycle. These four features are so important that each merits a
short description of its own.
• There are three subphyla under phylum
Chordata. Two of these subphy1a are small,
lack a vertebral column, and are of interest
primarily as borderline or first chordates
(protochordates，原索动物).
• Subphylum Vertebrata （脊椎动物亚门）：
Bony or cartilaginous vertebrae surround
spinal cord， notochord in all embryonic
stages and persists in some of the fish.
脊椎动物是和人类关系最密切的一类动物
1 人类本身就是一类特殊的脊椎动物
2 脊椎动物和人类的衣食紧密相关
3 提供了丰富的中医药
4 提供了满足各种研究需要的实验动物
5 要保护的大多数动物都是脊椎动物
6 给人类也带来很多危害
二 如何学好脊椎动物学
1以进化的观点来学习
2 形态特点和和生态环境紧密结合
3 理论教学和实验教学紧密结合
4 理论和实践紧密结合
5 以比较的方法来学习
进化地位
是所有动物中最高等的一门动物，
包括尾索动物、头索动物和脊椎动物。
脊索动物由无脊椎动物演化而来，
与棘皮动物和半索动物具有共同的祖先。
脊椎动物是神经系统获得最充分发
展的动物，并由此最后演化出具有自我
意识的人。
Characteristics of chordates
（脊索动物的特征）
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1） Notochord (a skeletal rod) present at some stage in life cycle
2） Nerve cord dorsal and tubular: anterior end of cord usually enlarged to
form brain
3） Pharyngeal gill slits present at some stage in life cycle and may or may
not be functional
4） A postanal tail usua1ly projecting beyond the anus at some stage and
may or not persist
5） Deuterostome
6） Heart ventral, with dorsal and ventral blood vessels; closed blood
system
7） Complete digestive system
8） Exoskeleton often present; well developed in some vertebrates
9） A cartilage or bony endoskeleton present in the majority of members
(vertebrates)
10） Bilateral symmetry; segmented body; three germ layers; coelom well
developed
脊索的出现在动物演化史上的意义
支持
保护
运动
• DORSAL TUBULAR NERVE CORD. In the invertebrate phyla the
nerve cord (often paired) is ventral to the alimentary canal and is solid,
but in the chordates the cord is dorsa1 to the alimentary canal and is
formed as a tube.The anterior end of this tube in vertebrates becomes
enlarged to form the brain. The hol1ow cord is produced by the
infoIding of ectodermal cells on the dorsal side of the body above the
notochord. Among the vertebrates the nerve cord lies in the neural
arches of the vertebrae, and the anterior brain is surrounded by a bony
or cartilaglnous cranlum.
• PHARYNGEAL GILL SLITS.
• Pharyngeal gill slits are perforated slitlike openings that lead from
the pharyngeal cavity to the outside. They are formed by the
invagination of the outside ectoderm and the evagination of the
endodermal lining of the pharynx. The two pockets break through
when they meet, to form the slit. In higher vertebrates these
pockets may not break through and only grooves are formed
instead of slits; all traces of them usually disappear. In forms that
use the slits for breathing, gills with blood vessels are attached to
the margins of the slits and make the gaseous exchange with the
water that enters the mouth and passes through the pharyngeal gill
slits. The slits have in their walls supporting frameworks of gill
bars. Primitive forms such as amphioxus have a large number of
slits, but only six or seven are the rule in the fish. The transitory
appearance of the slits in land vertebrates is often used as evidence
for evolution.
• 其他与无脊椎动物的区别
– 肛后尾：即尾在肛门后方；
– 闭管式循环系统；
– 心脏位于消化道腹面；
– 后口；
– 分节的肌节；
– 生物化学比较(无:精氨酸;脊椎:肌酸)
• 与部分无脊椎动物的共同点
– 后口
– 三胚层
– 真体腔
– 身体分节
– 两侧对称
ANCESTRY AND EVOLUTION
（脊索动物的祖先和进化）
• The great chasm between vertebrates and invertebrates seems
nearly as wide today as it did 150 years ago when zoologists
began serious speculation on the issue. The most serious
obstacle to progress is the almost total absence of fossi1
material for reconstructing lines of descent. The earliest
protovertebrates were in all probability soft-bodied forms that
stood litt1e chance of being preserved even under the most
ideal conditions. Consequently, speculations must come from
the study of living organisms, especially from an analysis of
how they develop. But well. over 600 million years have
elapsed since the vertebrates had their beginnings, and even the
highly conservative early developmental stages of animals
cannot be expected to remain unchanged over such an
enormous span of time.
EARLY THEORIES OF
CHORDATE EVOLUTION
• . The earliest speculations about vertebrate origins
understandably focused on the most successful, and in
may respects most advanced, of invertebrate groups.
• the Arthropoda. It was quickly recognized that if one
took an arthropod with its segmented body, ventral nerve
cord and dorsal heart and turned it over, one has the basic
plan of a vertebrate. The idea was first proposed by St.
Hilaire in 1818 and received detailed support by
subsequent zoologists.
• Later, in 1875 Semper and Dohrn independently
transferred the ancestral award to the Annelida, because
this group shares a basic body plan with the arthropods
and in addition has an excretory system that strikingly
resembles that of primitive vertebrates. The anne1idvertebrate theory continued to receive support as late as
1922 when the Dutch biologist Delsman published his
e1aborate arguments in a book. The principal difficulty
with the annelid theory was that an inverted annelid has
its brain and mouth in the wrong relative positions. The
annelid's nerve cord is ventra1 but connects to a
dorsal brain via circum pharyngeal connectives
through which the digestive tube passes (Fig. 16-4,
A). When an annelid is inverted, as was required for
the annelid-vertebrate theory, the mouth ends up on
top of the head, and the brain below.
ECHINODERM THEORY
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When, early in 20 century, further theorizing became rooted in developmenta1
patterns of animals, it immediate1y became apparent that on1y the echinoderms
deserved serious consideration as the vertebrate's ancestor.
Echinoderms and chordates belong to the deuterostome branch of the animal
kingdom in which the mouth is formed as a secondary opening and the
b1astopore of the gastrula becomes the anus.
The coelom of these two phyla is primitive1y enterocoelous:
Furthermore, there is a great resemb1ance between the bipinnaria larvae of
certain echinoderms and tornaria larvae of the hemichordates, a phylum
bearing some chordate characteristics. Both have similar ciliated bands in loops,
sensory cilia at the anterior end, and a complete digestive system of ventral
mouth and posterior anus.
Both echinoderm and chordate embryos show indeterminate cleavage
These characteristics are shared by brachiopods and pterobranchs (a
hemichordate group) as well as by echinoderms, protochordates, amphioxus,
and vertebrates. This is probably a natural grouping and almost certainly
indicates interrelationships. However the absence of biochemical support and
the uncertainty of fossil evidence in no way invalidates the embryologic
evidence for a remote kinship between echinoderms and chordates.
•
脊索动物的祖先可能类似尾索动物的
幼体，后向两个方向发展，一是经过变态，
成体为固着生活，具鳃裂作为取食和呼吸
器官；另一个是幼体期延长并适应新的生
活环境，不再变态，出现生殖腺并进行繁
殖（即幼体性成熟），进而发展出新的一
类动物，即具有脊索、背神经管和鳃裂的
自由运动的脊索动物。
系统发育研究的方法
• 1 形态学 形态特征的形似性
• 2 幼虫（系统发育） 幼虫具有的特征为
祖征，而成虫具有的为衍征。
• 3 分子生物学 蛋白质的氨基酸序列或核
酸序列分析。目前最流行的方法。
分子系统学
• 主要在分子水平，利用分子生物学手段探索生物遗传物质
----DNA的分子差异，探讨物种之间及大的类群之间的亲
缘关系，以促进合理的、自然的分类系统，揭示生命系统
的奥秘。在这一方面，20世纪90年代作了许多基础工作，
在脊索动物方面也作了大量工作。例如Russo等人（1996）
对11种脊椎动物的线粒体 DNA（mt DNA的完整序列）测
序分析。这些种的mtDNA包括13个编码蛋白质的基因。
这11种物种中共有的密码子的数目从52个到582个不等，
这是随基因的不同而不同的。对这些基因中的每一个都构
建了系统树，并对全部基因（3682个密码子）也构建了系
统树。他把这些树与真实树进行了比较。这里，我们考虑
的是用氨基酸序列，而不是核苷酸序列所获得的结果，因
为前者比后者获得的树更可信。
图3-1（A）已知的11种脊椎动物物种的系统发育树
（B）加入七鳃鳗和海胆的序列后推断的系统发育树。
图3-2 用线粒体细胞色素b基因的核苷酸和氨基酸序列构建的
11种脊椎动物的系统发育树。
TL：树长； PC：泊松校正距离； Kimura：Kimura距离
The Divesity of Chordates（第二
节 脊索动物门的多样性）
• . CHORDATE SUBPHYLA
• There are three subphyla under phylum Chordata.
Two of these subphy1a are small, lack a vertebral
column, and are of interest primarily as borderline
or first chordates (protochordates，原索动物).
Since these subphyla lack a cranium, they are also
referred to as Acrania（无头类）. The third
subphylum is provided with a vertebral column
and is cal1ed Vertebrata. Since this phylum has a
cranium, it is also called Craniata（有头类）.
• Protochordata (Acrania)
• Subphylum Urochordata （尾索动物亚门）：
(Tunicata海鞘). Notochord and nerve cord on1y in
free--swimming larva; adults sessile and encased in
tunic. Example:Molgula.
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Appendiculariae（尾海鞘纲）
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Ascidiacea（海鞘纲）
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Thaliacea(樽海鞘纲)
• Subphylum Cephalochordata（头索动物亚门）：
Notochord and nerve cord found aIong entire length
of body and persist throughout life; fishlike in form.
Example: Branchiostoma (Amphioxus文昌鱼).
• Cephalochorda（头索纲)
• Craniata
• Subphylum Vertebrata （脊椎动物亚门）：Bony or
cartilaginous vertebrae surround spinal cord，notochord in all
embryonic stages and persists in some of the fish. This
subphylum may also be divided into two great groups
(superclasses) according to whether they have jaws（颌）.
• Superclass Cyclostomata（圆口总纲）： (Agnatha无颌类).
Without true jaws or appendages. Example: Pltromyzon.
• Cyclostomata（圆口纲)
• Superclass Cnathostomata（有颌类） ： With jaws and
(usually) paired appendages. Example: Homo（人类）.
• Pisces （鱼纲） （软骨鱼纲Chondrichthyes，硬骨鱼纲
Osteichthyes）
• Amphibia （两栖纲 ）
• Reptilia （爬行纲）
• Aves （鸟 纲）
• Mammalia （哺乳纲）
中国和世界脊椎动物物种数比较
脊椎动物主要类群
圆口纲
软骨鱼纲
硬骨鱼纲
两栖纲
爬行纲
鸟纲
哺乳纲
上下颌
无颌类
附肢
鱼形类
有 颌 类 (颌 口
类)
四足类
胚膜
无羊膜类
羊膜类
体温
变温动物
恒温动物
Urochordata（ 尾索动物亚门）
Adult ascidian—Molgula(海鞘)
Molgula is globose in form and is
attached by its base to piles and
stones. Lining the test or tunic is a
membrane or mantle. On the
outside are two projections: the
incurrent and excurrent siphons.
Water enters the incurrent siphon
and passes into the pharynx
(branchial sac) through the mouth.
On the mid-ventral side of the
pharynx is a groove, the endostyle
（内柱）, which is ciliated and
secretes mucus. Food materia1 in
the water is entang1ed by the
mucus in this endostyle and
carried into the esophagus（食道）
and stomach. The intestine leads
to the anus near the excurrent
siphon.
• The water passes through the pharyngeal slits in the walls of the
pharynx into the atrial cavity（围鳃腔）. As the water passes
through the slits, respiration occurs.
• The circulatory system contains a ventral heart near the stomach and
two large vessels, one connected to each end of the heart. The action
of the heart is peculiar in that it drives the blood first in one direction
and then in the other. This reversal of blood flow is found in no other
animal.
• The excretory system is a type of nephridium（肾管） near the
intestine.
• The nervous system is restricted to a nerve ganglion and a few
nerves that lie on the dorsal side of the pharynx.
• A notochord is lacking.
• The animals are hermaphroditic（雌雄同体）, for both ovaries and
testes are found in the same anima1. Ducts lead from the gonads
close to the intestines and empty near the anus. The germ cells are
carried out the excurrent siphon into the surrounding water, where
cross-fertilization occurs.
• It will be seen that, of the four chief characteristics of chordates,
adult tunicates have only one, the pharyngeal glll slits. However, the
larval form gives away the secret of their true relationship.
Ascidian tadpole（海鞘幼虫）
• The tadpole larvae among the different ascidians vary in certain details, but the basic
plan is much the same in all.
• The deve1opment of the egg through the blastula and gastrula stages is somewhat
similar to that of amphioxus. After a development of about 2 days the embryo
hatches out into an elongated transparent larva about 1 to 5 mm. long. Its tail, four or
five times longer than its trunk, is provided with a slender cuticular fin and contains
the following structures:
• a notochord of vacuolated cells arranged in a single row; a hollow dorsal nerve
chord extending from the tip of the tail to the sensory vesicle and made up of small
cells; and a striated muscle band on each side of the notochord.
• In the larger head and trunk regions are found the three adhesive
papillae a digestive system of dorsal mouth, short esophagus, large
pharynx with endostyle and gill slits (which open into the atrium),
stomach, intestine, and anus (which opens into the atrium): the brain,
which is a continuation of the nerve cord of the tail: a sensory vesicle
containing an otolith for balance; and a dorsal median eye with lens
and pigmented cup. A coelom and a circulatory system are present, but
the heart is not formed until after metamorphosis. The larva does not
feed but swims around for some hours, during which time it is at first
positively phototactic(趋光性) and negatively geotactic but later
becomes negatively phototatic and positively geotactic. By its
adhesive papi1lae it now fastens itself vertically to some solid object
and then undergoes retrograde metamorphosis to become an adult. In
this process the tail is absorbed by phagocytes; the notochord, muscles,
and nervous system (except a trunk ganglion) degenerate; the
branchial sac enlarges with many gill slits; and the alimentary canal
and circulatory system (with a heart) enlarge and develop. The body
also undergoes a rotation so that the mouth and atrial openings
(siphons) are shifted to the upper unattached end. Gonads and ducts
arise in the mesoderm, and the who1e animal becomes enclosed in a
test or tunic.
• The evolutionary significance of the ascidian tadpole has already
been discussed.
• 2 幼体和变态
幼体具脊索动物 3 大特征，尾内有脊索；脊索背部
有中空的背神经管，神经管前端膨大成脑泡，并具眼点
和平衡器；消化道前端分化成咽，咽壁上有鳃裂。尾在
肛门后方，心脏位于腹面。
变态期间：尾部连同脊索逐渐萎缩消失，神经管退化成神
经节，感觉器官消失,裂数增加，体壁分泌被囊素形成
被囊，开始营固着生活。
这种从幼体至成体结构更为简单化的变态称为逆行
变态或退化变态(retrogressive metamorphosis)。
尾
索
动
物
分
类
尾海鞘纲(幼形纲)
海鞘纲
樽海鞘纲
Subphylum Cephalochordata（头索动物亚门）
• Amphioxus（文昌鱼）
• Subphylum Cephalochordata is the most interesting of all the
protochordates, for one of its members is the lancelet Branchiostoma
(Amphioxus) , one of the classic animals in zoology. This group is
found mainly on the sandy beaches of southern waters, where they
burrow in the sand, with the anterior-end projecting out. One
American species, B. virginiae. is found from Florida to the
Chesapeake Bay. Altogether there are 28 species, of which 4 are North
American, scattered over the world. They can swim in open water by
swift lateral movements of the body. B. belcheri is found in China.
• Amphioxus is especial1y interesting, for it has the four
distinctive characteristics of chordates in simple form, and in
other ways it may be considered a blueprint of the phylum. It has
a 1ong, slender, laterally compressed body 2 to 3 inches long,
with both ends pointed. There is a long dorsal fin, which passes
around the tail end to form the caudal fin. A short ventral fin is
also found. These fins are reinforced by fin rays of connective
tissue. The ventral side of the body is flattened and bears along
each side a metapleural fold（腹褶）. There are three openings
to the outside; the ventral anterior mouth, the anus near the base
of the caudal fin, and the atriopore（腹孔） just anterior to the
ventral fin.
• The body is covered with a soft epithelium one layer thick
resting upon some connective tissue. The notochord, which
extends almost the entire length of the body, is made up of
cells and gelatinous substance enclosed in a sheath of
connective tissue. Above the notochord is the tubular dorsal
nerve cord, with a slight dilation at the anterior end known
as the cerebral vesicle.
• Along each side of the body and tail are the numerous <
shaped myotomes（肌节）, or muscles, which have a
metameric arrangement. The myotomes are separated from
each other by myosepta of connective tissue. The myotomes
of the two sides alternate with each other. The anterior end
of the body is called the rostrum. Just back of this and
slightly below is a median opening surrounded by a
membrane, the oral hood, which bears some twenty oral
tentacles (buccal cirri 触须). The oral hood encloses the
chamber known as the vestibule, at the bottom of which lies
the true mouth with a membrane, the velum, around it.
Around the mouth are 12 velar tentacles. The cirri and
tentacles serve to strain out large particles and have sensory
functions. Ciliated patches on the walls of the buccal cavity
in front of the velum produce a rotating effect and are called
the wheel organ, which propels water currents.
• On the dorsal side of the oral hood is hatschek's groove and pit, an
embryonic relic from the first coelomic sac on the left. It may be
homologous with the pituitary of vertebrates. Just behind the mouth is
the large compressed pharynx with more than a hundred pairs of gill
slits, which act as strainers in filter feeding as well as in respiration.
From the pharynx the narrow tubular intestine extends backward to
the anus.
• On the ventral side of the intestine is a large diverticu1um, the hepatic
cecum.
• The coelom is reduced and is confined to the region above the
pharynx and around the intestine . Connecting the coelom to the
atrium are about a hundred pairs of ciliated nephridia of the
solenocyte type, a modified kind of flame cell. The big cavity around
the pharynx is the atrium; it is lined with ectoderm and is therefore not
a coelom. The pharynx has a mid-dorsal groove, the hyperbranchial
(epipharyngeal) groove, and a mid-ventral one, known as the
endostyle. Both of these grooves are lined with cllia and gland cells.
Food is entangled by the mucus of the endostyle and is carried to the
intestinei the water passes through the gill slits into the atrium and
gives up oxygen to the blood vessels in the gill bars.
• Although there is no heart, the blood system similar to that
of higher chordates . The blood moves posteriorly in the
dorsal aorta and anteriorly in the ventral aorta; a hepatic
portal vein leads from the intestines to the liver. Blood is
propelled by contractions of the ventral aorta and is carried
to the dorsal aorta by vessels in the gill bars. The blood is
almost colprless, with a few red corpuscles.
• The nervous system is above the notochord an. consists of
a single dorsal nerve cord, which is hollow. This nerve
cord gives off a pair of "nerves" alternately to each body
segment, or myotome, as dorsal and ventral roots.
Although this arrangement resembles the vertebrate spinal
cord, the ventral root actually does not contain nerves but
rather the drawn-out extensions of myotomic muscle
fibers.
• Ciliated cells with sensory functions are found in various
parts of the body. Sexes are separate, and each sex has
about 25 pairs of gonads located on the wall of the atrium.
The sex cells are set free in the atrial cavity and pass out
the atriopore to the outside, where fertilization occurs.
Cleavage is total (holoblastic) and a gastrula is formed by
invagination. The larva hatches soon after deposition and
gradually assumes the shape of the adult.
• No other chordate shows so well the basic diagnostic
chordate characteristics as does the amphioxus. Not only are
the four chief characters of chordatese，dorsal nerve cord,
notochord, pharyngeal gill slits, and postanal tail--well
represented but also secondary characteristics, such as liver
diverticulum, hepatic portal system, and the beginning of a
ventral heart. Indicative also of the condition in vertebrates
is the much thicker dorsal portion of the muscular layer.
This is in contrast to the invertebrate phyla in which the
muscu1ar layer is about the same thickness around the body
cavity. The metameric arrangement of the muscles suggests
a similar p1an in the embryos of vertebrates. Just where it is
placed in the evolutionary blueprint of the chordates and
the vertebrates is a controversial point.
头索动物是典型的桥梁动物：
除终生具有以上脊索动物的三个主要特征外，还具有
A.与脊椎动物相似的特征
具肌节、有奇鳍、肛后尾、闭管式循环系统，其胚
胎发育和三个胚层的分化也与脊椎动物相似。
B.比脊椎动物原始的特征：
无头、无骨骼、无心脏、排泄器官为肾管。
C.比无脊椎动物进化的特征：
躯体两侧有一对腹褶，为脊椎动物成对附肢的雏形。
思考题
1.脊索动物门有何共同特征？与无脊椎动物
比较有何功能和进化意义？
2.何为逆行变态？了解逆行变态有何意义？
3.文昌鱼为什么被称为头索动物？简述文昌
鱼结构的原始性、特化性和进步性。
4.试述脊索动物的起源。
5.试述脊索动物门中亚门和纲的分类及各类
群的主要特征。